The present invention generally relates to superconducting circuits and more particularly to a system for eliminating trapped magnetic flux from a superconducting circuit.
Superconducting circuits such as SQUIDs (superconducting quantum interference device) or Josephson circuits are studied intensively. The former is studied in relation to magnetic detectors and sensors that have an ultra-high sensitivity while the latter is studied in relation to the ultra-fast logic devices and computers.
In these superconducting circuits that use the superconducting phenomenon, there arises a problem in that the external magnetic flux, such as the one formed by the earth's magnetic field, is often trapped in a superconductor when the superconducting circuit is cooled across a critical temperature, above which the superconductor assumes the normal conduction state, and below which the superconductor assumes the superconduction state. When such a trap of magnetic flux occurs in the superconductor that forms the essential part of the superconducting device, the operation of the device is influenced unwantedly by the trapped magnetic flux. For example, the threshold characteristic of the SQUIDs may be modified upon the trapping of the magnetic field as will be described later.
In principle, such a trap of the magnetic flux would not occur when the superconductor is formed as a thin strip and cooled consecutively from one end to the other. However, the cooling is generally made for the entire superconductor body in one step by immersing in a liquid helium, and there is a tendency that a region of normal conduction remains in the region of the superconduction like an isolated island due to the fluctuation of temperature at the time of crossing of the critical temperature. Generally, such a fluctuation of the temperature cannot be eliminated completely while the minute fluctuation is enough to cause the trapping of the magnetic flux.
Conventionally, efforts are made to eliminate such a trap of the magnetic flux by using a magnetic shield such as a permalloy enclosure or superconducting enclosure to eliminate the external magnetic field. In this approach, the cooling of the superconducting circuit across the critical temperature is made in such a magnetic shield enclosure. Alternately, there is proposed a use of so-called moat structure wherein a groove called "moat" is provided on the ground plane of the superconducting device to surround the essential part of the device. In the latter construction, exclusion of the magnetic flux is possible by setting the area S of the essential part of the device to satisfy a relation S.times.B&lt;.phi..sub.0 /2, where .phi..sub.0 represents the flux quantum in the superconductor. It should be noted that the magnetic flux is quantized in the superconducting materials. In the latter construction, one can expel the magnetic flux from the superconductor into the moat that is formed to surround the essential part of the device. The foregoing approaches, however, can provide only a limited success in eliminating the trap of the magnetic flux particularly in the large sized superconductor circuits such as the SQUID, even when the former and the latter constructions are combined.
There is another approach to eliminate the magnetic flux known as a heat flash process, wherein the superconductor is heated to a temperature above the critical temperature by using a resistance heater formed in or adjacent to the superconducting material. By cooling the superconductor again to below the critical temperature, there occurs a random fluctuation of the temperature that is induced naturally, and such a fluctuation of temperature causes a modified distribution of the flux quantum in the superconducting material. By repeatedly heating and cooling the superconductor, the chance that the flux quantum is moved to the outside of the superconductor or at least displaced from the critical part of the superconducting circuit is increased and it is expected that the residual flux quantum is gradually eliminated from the superconducting circuit. In this heat flash approach, however, the heating of the superconductor has to be made with utmost care, particularly when a resistance heater is employed for the purpose, such that the magnetic flux accompanying the electric current that is used to drive the resistance heater does not enter the superconducting material. Otherwise, new flux quantum would be trapped in the superconducting material and the adversary effect of the residual flux quantum is not reduced but enhanced. Further, the conventional resistance heaters used for this purpose generally cause a uniform heating of the superconductor. Thereby, the movement of the flux quantum occurs only as a matter of chance, and the expelling of the flux quantum to the outside of the superconductor is not guaranteed. Thus, there usually remains substantial amount of flux quanta in the superconductor even if the process is repeated for a number of times.
Further, there is another known approach, described in the Japanese Patent Publication No. 1-42512, to eliminate the flux quantum from the superconductor forming a superconducting circuit, wherein the heating of the superconductor to a temperature above the critical temperature is made by a laser beam irradiation. In this approach, the laser beam is scanned over the superconductor held in the superconducting state, starting from one end to the other end, such that a region of normal conduction state, formed as a result of the heating by the laser beam, is moved from the foregoing one end to the other end. Thereby, the region of the normal conduction collects the flux quantum in the region that is swept by the laser beam and the flux quantum thus collected is transported to the outside of the superconductor.
In operation, the location of the superconductor containing the residual flux quantum is identified at first by using a SQUID detector, and the laser beam is directed to such a region thus detected. The laser beam is then moved along the surface of the superconductor until the beam spot goes off. Further, the foregoing process is repeated until the flux quantum is eliminated from the superconductor.
This approach, however, requires a complex scanning mechanism of the laser beam in addition to the laser apparatus itself. Further, the scanning mechanism can generally provide only the linear scanning and the efficiency of the flux elimination is small. Furthermore, the shape of the superconductor that can be applicable to this known process is limited even when the scanning mechanism is combined with a rotation mechanism to irradiate the superconductor from all directions.